Hybrid-electric drives are currently used in many fields. In particular, internal combustion engines and generators are combined with one another for the purpose of generating electricity. In this case, the generator is intended to be designed in the simplest manner possible and to be as compact and robust as possible.
Permanent-magnet synchronous machines (PMSMs) appear to be particularly suitable for hybrid-electric drives on account of their high power density. However, precise determination of the rotor position is required for efficient operation.
To determine the rotor position, inductive rotary encoders, so-called resolvers, which can identify the rotor position very accurately, are usually used. With knowledge of the rotor position, field-oriented regulation of the synchronous machine can be achieved in rotational speed ranges close to stationary and across the entire operating range of the engine.
However, resolvers together with associated electronics are expensive and sensitive. Sensorless solutions, in contrast, function only to a limited extent: the combination of a synchronous machine with an internal combustion engine without its own starter, which is intended to be cranked up by the synchronous machine for starting, thus poses additional demands. In this case, the highly fluctuating torque also appears in the cylinders due to the compression, wherein in each case peaks in the countertorque arise only temporarily. In many cases, this prevents successful purely controlled run-up.
A similar problem is sensorless operation of piston compressors using a PMSM. Conventional sensorless methods, for example the estimation of the rotor position by an observer model in addition to field-oriented regulation of the synchronous machine, typically deliver sufficiently accurate information for stable field-oriented regulation of the synchronous machine up to the rated torque only for rotational speeds from about 10% of the maximum rotational speed.
In the case of lower rotational speeds, sensorless estimation of the rotor position is difficult or impossible on account of the only low induced voltage. In these rotational speed ranges, the instantaneous rotor position can be identified to a certain degree of accuracy at certain times by suitable test pulses, but this is insufficient in most cases. The torque-generating current also has to be switched off during the test pulses.
A purely controlled run-up in a rotational speed range in which the estimation of the rotor position functions sufficiently accurately by an observer model is problematic. The torque that can be achieved in this operating range is usually only a fraction of the normal rated torque of the engine, since, even at low countertorques, “tilting” of the engine cannot be avoided and hence the torque immediately drops. Targeted setting of the torque is likewise not possible in this operating mode. For applications with low countertorques at low rotational speeds, this is sufficient in individual cases at best.